One pervasive feature of daily life in modern industrialized societies is the use of paper products for a variety of purposes. Paper towels, facial tissues, toilet tissue, and the like are in almost constant use. The large demand for such paper products has created a demand for improved versions of the products and of the methods of their manufacture. Despite great strides in paper making, research and development efforts continue to be aimed at improving both the products and their processes of manufacture.
Paper products such as paper towels, facial tissues, toilet tissue, and the like are made from one or more webs of tissue paper. If the products are to perform their intended tasks and to find wide acceptance, they, and the tissue paper webs from which they are made, must exhibit certain physical characteristics. Among the more important of these characteristics are strength, softness, and absorbency.
Strength is the ability of a paper web to retain its physical integrity during use.
Softness is the pleasing tactile sensation one perceives when they crumple the paper in their hands and when they use the paper for its intended purposes.
Absorbency is the characteristic of the paper which allows it to take up and retain fluids, particularly water and aqueous solutions and suspensions. In evaluating the absorbency of paper, not only is the absolute quantity of fluid a given amount of paper will hold significant, but the rate at which the paper will absorb the fluid is also important. In addition, when the paper is formed into a product such as a towel or wipe, the ability of the paper to cause a fluid to be taken up into the paper and thereby leave a dry wiped surface is also important.
Processes for the manufacturing of paper products for use in tissue, toweling and sanitary products generally involve the preparation of an aqueous slurry of paper fibers and then subsequently removing the water from the slurry while contemporaneously rearranging the fibers in the slurry to form a paper web. Various types of machinery can be employed to assist in the dewatering process.
Currently, most manufacturing processes either employ machines which are known as Fourdrinier wire papermaking machines or machines which are known as twin (Fourdrinier) wire papermachines. In Fourdrinier wire papermaking machines, the paper slurry is fed onto the top surface of a traveling endless belt, which serves as the initial papermaking surface of the machine. In twin wire machines, the slurry is deposited between a pair of converging Fourdrinier wires in which the initial dewatering and rearranging in the papermaking process are carried out.
After the initial forming of the paper web on the Fourdrinier wire or wires, both types of machines generally carry the paper web through a drying process or processes on another fabric in the form of an endless belt which is often different from the Fourdrinier wire or wires. This other fabric is sometimes referred to as a drying fabric. Numerous arrangements of the Fourdrinier wire(s) and the drying fabric(s) as well as the drying process(es) have been used successfully and somewhat less than successfully. The drying process(es) can involve mechanical compaction of the paper web, vacuum dewatering, drying by blowing heated air through the paper web, and other types of processes.
As seen above, papermaking belts or fabrics carry various names depending on their intended use. Fourdrinier wires, also known as Fourdrinier belts, forming wires, or forming fabrics are those which are used in the initial forming zone of the papermaking machine. Dryer fabrics as noted above, are those which carry the paper web through the drying operation of the papermaking machine. Various other types of belts or fabrics are possible also. Most papermaking belts employed in the past are commonly formed from a length of woven fabric the ends of which have been joined together in a seam to form an endless belt. Woven papermaking fabrics generally comprise a plurality of spaced longitudinal warp threads and a plurality of spaced transverse weft threads which have been woven together in a specific weaving pattern. Prior belts have included single layer (of warp and weft threads) fabrics, multilayered fabrics, and fabrics with several layers each of which comprises interwoven warp and weft threads. Initially, the threads of papermaking fabrics were made from wires comprised of materials such as phosphor bronze, bronze, stainless steel, brass or combinations thereof. Often various materials were placed on top of and affixed to the fabrics to attempt to make the dewatering process more efficient. Recently, in the papermaking field, it has been found that synthetic materials may be used in whole or part to produce the underlying wire structures, which are superior in quality to the forming wires made of metal threads. Such synthetic materials have included Nylon, polyesters, acrylic fibers and copolymers. While many different processes, fabrics, and arrangements of these fabrics have been used, only certain of these processes, fabrics, and arrangements of these fabrics have resulted in commercially successful paper products.
An example of paper webs which have been widely accepted by the consuming public are those made by the process described in U.S. Pat. No. 3,301,746 issued to Sanford and Sisson on Jan. 31, 1967. Other widely accepted paper products are made by the process described in U.S. Pat. No. 3,994,771 issued to Morgan and Rich on Nov. 30, 1976. Despite the high quality of products made by these two processes, however, the search for still improved products has, as noted above, continued.
Another commercially significant improvement was made upon the above paper webs by the process described in U.S. Pat. No. 4,529,480 issued to Trokhan on July 16, 1985, which is incorporated by reference herein. The improvement included utilizing a papermaking belt (which was termed a "deflection member") comprised of a foraminous woven member which was surrounded by a hardened photosensitive resin framework. The resin framework was provided with a plurality of discrete, isolated, channels known as "deflection conduits." The process in which this deflection member was used involved, among other steps, associating an embryonic web of papermaking fibers with the top surface of the deflection member and applying a vacuum or other fluid pressure differential to the web from the backside (machine-contacting side) of the deflection member. The papermaking belt used in this process was termed a "deflection member" because the papermaking fibers would be deflected into and rearranged into the deflection conduits of the hardened resin framework upon the application of the fluid pressure differential. By utilizing the aforementioned improved papermaking process, as noted below, it was finally possible to create paper having certain desired preselected characteristics.
The deflection member described in the aforementioned patent issued to Trokhan was made by the process described in U.S. Pat. No. 4,514,345, issued in the name of Johnson, et al., which is the incorporated by reference herein. The process described in the Johnson, et al. patent includes the steps of: 1) coating the foraminous woven element with a photosensitive resin; 2) controlling the thickness of the photosensitive resin to a preselected value; 3) exposing the resin to a light having an activating wave length through a mask having opaque and transparent regions; and 4) removing the uncured resin. This process produced a deflection member with a framework which had a paper web-contacting surface and a machine-contacting surface that were each provided with a network pattern surrounding the conduits which was essentially monoplanar or smooth.
The paper produced using the process disclosed in U.S. Pat. No. 4,529,480 is described in U.S. Pat. No. 4,637,859, issued in the name of Trokhan, which is incorporated herein by reference. This paper is characterized by having two physically distinct regions distributed across its surfaces. One of the regions is a continuous network region which has a relatively high density and high intrinsic strength. The other region is one which is comprised of a plurality of domes which are completely encircled by the network region. The domes in the latter region have relatively low densities and relatively low intrinsic strengths compared to the network region.
The paper produced by the process described in U.S. Pat. No. 4,529,480 was actually stronger, softer, and more absorbent than the paper produced by the preceding processes as a result of several factors. The strength of the paper produced was increased as a result of the relatively high intrinsic strength provided by the network region. The softness of the paper produced was increased as a result of the provision of the plurality of low density domes across the surface of the paper. The absolute quantity of fluid the paper would hold (one of the key factors in determining the absorbency of the paper) was increased due to the fact that the overall density of the paper was reduced.
Although the aforementioned improved process worked quite well, it has been found that when the deflection member of the above-described process passed over vacuum dewatering equipment used in the papermaking process, certain undesirable events occurred. Of most concern was the fact that a large number of partially dewatered fibers in the paper web would pass completely through the deflection member. This would lead to the undesirable result of clogging the vacuum dewatering machinery with the more mobile paper fibers. Another undesirable occurrence was the tendency of these mobile paper fibers to accumulate on the dewatering machinery to the extent of producing clumps of fibers on the machinery. This accumulation of fibers would cause the previous papermaking belts which had smooth backsides to wrinkle and develop folds, particularly longitudinal folds, after they repeatedly traveled over the dewatering machinery during the papermaking process, which in turn would not only result in severe problems with the moisture and physical property profiles of the paper produced, but would result in the eventual failure of the papermaking belt.
The significance of the difficulties experienced with these prior belts was increased by the relatively high cost of the belts. In most cases, manufacturing the foraminous woven element which was incorporated into these belts required (and still requires) expensive textile processing operations, including the use of large and costly looms. Also, substantial quantities of relatively expensive filaments are incorporated into these woven elements. The cost of the belts is further increased when high heat resistant filaments properties are employed, which is generally necessary for belts which pass through a drying operation.
In addition to the cost of the belt itself, the failure of a papermaking belt will also have serious implications on the efficiency of the papermaking process. A high frequency of paper machine belt failures can substantially affect the economies of a paper manufacturing business due to the loss of the use of the expensive papermaking machinery (that is, the machine "downtime") during the time a replacement belt is being fitted on the papermaking machine.
At the time the papermaking process described in U.S. Pat. No. 4,529,480 was developed it was believed that the network formed in the lower surface of the resinous framework (the machine-contacting surface) had to be essentially planar in order to achieve the desired suddenness of application of vacuum pressure needed to deflect and rearrange the fibers into the deflection conduits to form the dome regions in the improved paper.
While not wishing to be bound by any theory, it is now believed that the problems which developed when using the prior smooth backsided papermaking belts may have been at least partially the result of the extremely sudden application of vacuum pressure to the paper web when it passed over the vacuum dewatering machinery. It is believed that the prior smooth backsided papermaking belts would actually temporarily create a seal over the vacuum source. Then, when the open channels (the deflection conduits) of the papermaking belt of the prior type were encountered, the vacuum pressure would be applied to the water laden, highly mobile fibers in the fibrous web situated on top of the resin framework in an extremely sudden fashion. This sudden application of vacuum pressure is believed to have caused the sudden deflection of the mobile fibers which was sufficient to allow them to pass completely through the papermaking belt. It is also believed that this sudden application of vacuum pressure and migration of fibers would account for pin-sized holes in the dome regions of the finished paper, which in some, but not all cases, are undesirable.
Another theory for the excessive accumulation of paper fibers on the surfaces of the vacuum dewatering equipment is that the prior smooth backsided papermaking belts did not have adequate surface texture on their backsides. It is believed that a certain amount of surface texture is necessary to enable such resin-coated belts to remove the paper fibers which accumulate on the vacuum dewatering equipment by the abrasive action of such a belt traveling over the vacuum dewatering equipment.
As a result, a need exists for an improved papermaking process which will not be plagued by the undesirable buildup of these mobile papermaking fibers on the vacuum dewatering machinery employed in the process. A need, therefore, also exists for an improved papermaking belt and a method of making the same which will eliminate the foregoing problems caused by utilizing a papermaking belt made by the prior processes.
Therefore, it is an object of the present invention to provide an improved papermaking process in which the migration of the aforementioned mobile paper fibers is substantially reduced, or eliminated.
It is also an object of the present invention to provide a papermaking belt which will substantially reduce the previous problem of the buildup of paper fibers on the vacuum dewatering machinery which was associated with the prior resin coated papermaking belts.
It is another object of the present invention to reduce the folding and subsequent failures of the papermaking belts due to the accumulation of paper fibers on the surface of the vacuum dewatering equipment employed in the papermaking process.
It is also an object of the present invention to develop a papermaking process which will result in the elimination of the pin-sized holes in the dome regions of the finished paper web (unless such holes are a desirable characteristic for the particular paper being produced).
It is also an object of the present invention to provide a papermaking belt which has passageways that provide surface texture irregularities on the backside of the belt and a method of making this belt in which these passageways can be imparted to the belt without sacrificing the strength of the entire papermaking belt.
It is a further object of the present invention to provide a papermaking belt, which when employed in the papermaking process of the present invention will have a longer life than prior papermaking belts, and a method of making this papermaking belt which is cost effective.
These and other objects of the present invention will be more readily apparent when considered in reference to the following description and when taken in conjunction with the accompanying drawings.